How Diamonds Are Polished

Processes to transform a diamond in the rough into the exquisite brilliant of commerce

Like a skilled surgeon the diamond lapidary performs the delicate operation which is known as “slitting.” Just the right amount of the stone, no more and no less, must be split away. The intent expression on the face of the operator bears witness to the momentous effect of the slight blow he is about to strike on the steel knife edge which he holds in his left hand.

Even the refuse from this operation is valuable and must be saved. As the lapidary rough-shapes his diamonds, rubbing or “bruting” the one on the end of the long stick which he holds under his right arm, against the other on the rapidly turning spindle which is driven by the belt, the dust and fine fragments fall into the little box shown in the centre of the picture. Only the protruding end of the stick is visible in the illustration.

The surface irregularities, together with any superficial flaws or dark patches, are first split away from the stone, which breaks naturally along smooth, even surfaces parallel to the natural faces of the crystal. To accomplish this our diamond is firmly cemented to the end of a wooden stick which in turn is made fast in an upright position, and thereupon with the sharp corner of a diamond fragment a deep scratch is made in the surface of the stone. A knife edge is then held in the right position on the scratch, and a sharp blow with a light tool on the back of the knife edge suffices to remove the undesirable flake, leaving the surface bright and very smooth. Sometimes, when the stone is large, it is of advantage to saw it into two or more pieces so as to save as much as possible of the weight in cut diamonds. This is accomplished with a thin disk of bronze, about four inches in diameter, revolving very rapidly and having its edge charged with diamond dust at the beginning of the sawing. As the saw bites into the stone, it keeps recharging itself with the diamond sawdust. It takes many hours for this little “buzz saw” to eat its way through half an inch of diamond, but the finished product is so valuable that a day or so of labor makes little impression on the cost sheet.

The rough shaping of the diamond is done through an operation called “bruting,” which consists of wearing away the corners by rubbing one stone against another. Formerly this was a strictly manual process, the two diamonds being mounted on sticks held in either hand by the lapidary. Even into the ancient and conservative art of diamond cutting, however, some mechanical improvements have made their way, and now in most of the shops a rapidly twirling spindle takes the place of one of the hand sticks. The remaining stick has grown in length to suit the modern method. It is now about two feet long and can be firmly grasped with both hands and held in a rest so that the diamond it bears at its end can be rubbed against its fellow, which is spinning around in front of it.

Having rough-shaped our diamond we now come to the finishing operation, the producing of the facets which give brilliancy and sparkle to it, an operation which is technically known as polishing. The holder of the stone during the polishing consists of a small metal cup on a long stem, which is called a “dop” and much resembles a tulip, which famous Dutch flower may have suggested its shape. A solder composed of one part tin and three parts lead is placed in the dop and heated until soft.

The diamond is then embedded in the solder with the portion of the stone on which the desired facet is to be cut placed uppermost and almost completely surrounded by the solder. When the diamond has been properly adjusted in the dop, it is plunged in cold water to cool and harden the solder. Such drastic treatment would cause less aristocratic stones promptly to fly to pieces, but not so with the diamond; the high heat conductivity of this remarkable substance permits it to submit to the sudden change of temperature without there resulting in it even the slightest flaw.

The dop is now fastened by means of its stem in a heavy iron arm called the tongs, in such a way as to bring the position of the facet to be cut exactly undermost when it is placed in contact with the polishing wheel or lap. The latter is made of soft iron and turns horizontally at the rate of about one thousand revolutions a minute. Diamond dust, mixed with olive oil, is fed to this wheel and the diamond is held in contact with it by weight of the tongs, aided by slabs of lead placed upon the latter. Several hours are required to cut one facet, then the stone is readjusted for another one, and so on until all of the fifty-eight little facets in which lies the secret of the brilliancy of the jewel are produced.

To appreciate the exquisite skill and infinite patience involved in this apparently simple process we have only to look at the gem on our finger, sending forth its magical fires, and to note the symmetry and regularity of shape of each of its tiny, glittering sides. And when we remember that to produce these rainbow-like rays each must have exactly the right tilt with respect to its neighbors, we realize that a cut diamond is not only a wonderful product of nature but a marvelous work of art.

Before the introduction of methods of diamond cutting in the fifteenth century, diamonds, when used in jewelry, were set with four of the eight sides of the octahedron or double pyramid projecting from the setting. This presented the aspect of a four-sided pyramid, and the exposed faces or facets were sometimes polished. The next step in the evolution of the modern form of diamond cutting was the production of a flat “table” on the exposed point by rubbing or “bruting” two crystals together. Thus we have the origin of the table facet as it is known today. In the early seventeenth century when the art of the diamond cutter had somewhat advanced, a more symmetrical outline for the stone was obtained by cutting away the four edges of the pyramid above the setting, which of course necessitated the equal cutting away of the four edges below.